Seven Palestinians die while picnicking on beach

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JERUSALEM (CNN) -- An Israeli navy gunboat fired shells onto a northern Gaza beach Friday, killing at least seven people and prompting the military wing of Hamas to call off a 16-month-old cease-fire with Israel.

The Israel Defense Forces said it was firing on rocket-launching areas in Gaza after a militant attack earlier in the day. The target areas were believed to be uninhabited, a representative said.

A spokesman for Hamas, Sami Abu Zuhri, said it was "impossible to remain silent" after viewing "terrifying pictures of the women and children" on the beach.

A mass demonstration erupted in Gaza City, and protesters Friday evening were demanding revenge for recent attacks that include the beach shelling and Thursday's killing of a Hamas official in an Israeli missile strike.

Palestinians were picnicking when the Israeli shells slammed into the beach, Palestinian medical sources said. About 20 people were injured in the attack, Palestinian security sources said.

An apology was initially issued to the Israeli newspaper Haaretz in which the IDF said it "regretted the strike on innocents," but Lt. Gen. Dan Halutz, the IDF's chief of general staff, later said the military is investigating the possibility that it was not naval artillery that struck the beach.

The IDF halted firing pending an investigation.

Haaretz quoted Palestinian Authority President Mahmoud Abbas as condemning the killings as a "bloody massacre."

Video footage from the beach showed ambulance workers carrying away injured people on stretchers and a young girl screaming for a parent.

Hamas has stuck to the cease-fire it announced in February 2005, but other groups did not sign on and have continued attacks against Israel. Earlier this year, Hamas won the Palestinian elections.

Hamas and its military wing Izzedine al Qassam has admitted responsibility for terrorist attacks against Israeli civilians as well as attacks against the Israeli military. Israel and the U.S. State Department consider Hamas a terrorist organization, though it also operates an extensive social services network in the territories.

Five militants also died earlier Friday in two Israeli strikes in Gaza, Palestinian sources said.

The beach is in the Palestinian village of Beit Lahya. The IDF said militants launched a Qassam rocket from the village that landed in Israel but caused no damage or casualties.

The militants got into a car, which the Israeli air force then attacked, according to the IDF. A large explosion followed, killing three people. The IDF said the blast was likely due to explosives in the car.

The three dead were members of the Popular Resistance Committees, a coalition of militant groups, Palestinian security sources said. Palestinian sources said the dead included two brothers and a cousin.

After the attack in Beit Lahya, a car was hit at Jabalya refuge camp, killing two militants, Palestinian sources said.

Last summer, under then-Prime Minister Ariel Sharon, Israel withdrew from Gaza, and Israeli forces forcibly removed about 8,000 settlers from Gaza and four small areas of the West Bank.